Press Forging of Magnesium Alloy AZ31 Sheets

Transcription

1 Materials Science Forum Online: ISSN: , Vols , pp doi: / Trans Tech Publications, Switzerland Press Forging of Magnesium Alloy AZ31 Sheets Shi-Hong Zhang*, Zhang-Gang Li, Yong-Chao Xu, Li-mei Ren, Zhong-Tang Wang, Li-Xin Zhou Institute of Metal Research, Chinese Academy of Sciences, Shenyang , China Abstract: Press forging of magnesium alloy AZ31 sheets was investigated in this paper. The typical component, a rectangular box with bosses at the bottom was formed. The experimental results show that the magnesium alloy sheets are suitable for press forging. The bosses and the rectangular box can be formed synchronously for 2 mm or 3 mm thick sheets when the punch temperature is 250 C. By experimentation and numerical simulation, the effects of process parameters on material flow were analyzed, including the temperature, the die shape, the blank size, the lubrication manners and the friction condition. Keywords: magnesium alloy; press forging; boss; rectangular box; finite element method 1. Introduction Recently research on stamp forming of magnesium alloy sheets is concentrated on warm deep drawing and hydroforming processes [1-4]. However, the thickness of the sheets cannot be changed remarkably. For example, the bosses and the ribs appear in the 3C components cannot be formed. Although die-casting process can be used to form these kinds of complicated shaped components, the surface of the products is not good enough and the accuracy is low, and parts with wall thickness less than 0.6 mm is not suitable for forming with casting process. As a new forming process of magnesium alloy sheets, press forging is applicable of manufacturing these components. It has the advantages of high production efficiency and surface quality, which can meet the requirements of the 3C components. So press forging process of Mg sheets at elevated temperatures was researched in the present paper by experimentation and FEM. 2. Press forging experiments 2.1 Shape of the formed part and the punch The designed shape of the rectangular box after press forging is shown in Fig.1. The lengths of the long side and the short side are 31.6 mm and 21.6 mm, respectively. The wall thickness is 0.7 mm. There are 4 bosses in the rectangular box, their shapes are conical. The maximal diameter at the bottom of the boss is 3.5 mm, and the minimal at the top is 2.5 mm. boss short si de l ong si de Fig.1 The designed shape of the rectangular box after press forging Two types of punches were used in the research work, one is the common type (punch A), and the other (punch B) is designed with the outflow track and constraint wall, which will improve the All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-12/05/16,17:00:16)

2 1754 THERMEC 2006 formability of the four corners of the box effectively. The shape of the punch is shown in Fig.2. The depth of the punch holes corresponding to the bosses is 5 mm. (a) punch A (b)punch B Fig.2 Punch shapes 2.2 Materials and characterization Commercial magnesium alloy AZ31B (nominally 3wt% Al, 1wt% Zn, balance Mg) sheets are used in the research work. The sheets are hot-rolled and the initial thickness is 20 mm. Cylindrical specimens of 8 mm in diameter and 15 mm in height are cut and machined for hot compression tests, which were carried out on a Gleeble 1500 thermo-mechanical simulator at constant strain rates in the range s 1 and constant temperatures in the range C.The test data are recorded during the full test and converted to true stress strain curves shown in Fig.3. For press forging experiments, the sheet blanks are 20 mm in width, 30 mm in length and 2 mm or 3 mm in thickness. Fig.3 The curves of stress strain at 250 C 2.3 Press forging experiments A universal hydraulic press was used in this research with a capacity of 1MN. Before forming, the blank surfaces are polished and lubricated with graphite that may improve the lubrication condition during metal forming and reduce wear of the dies. There are two ways to lubricate the blanks, the first being that the top-surface and bottom-surface are both fully lubricated, the second being that only the top-surface is fully lubricated and the bottom-surface is not lubricated or is lubricated a little only. The die and the punch are heated to C by using cartridge heaters. In order to prevent the heat emission, cartridge heaters are wrapped up with asbestos. The blank at room temperature is inserted into the dies, and then it is directly heated by the high-temperature tools [5] because of the high rate heat transfer of AZ31B. The punch speed is 0.2 mm/s constantly, and the strokes for the blank of 2 mm thickness and 3 mm thickness are 1.4 mm and 1.9 mm, respectively.

3 Materials Science Forum Vols Experimental results Fig.4 shows the experimental results of 2 mm thick blank after press forging at 250 C using punch A. Fig.4(a) shows the phenomenon of the twist and flow through, respectively, which happened at the lubrication condition that the bottom-surface and the top-surface were both fully lubricated. Fig.4(b) shows a good formed part that only the top-surface was fully lubricated and the bottom-surface was lubricated a little only. Fig.4(c) is the photograph showing good formed parts with different punch displacements. After press forging, the minimal thickness of the bottom is 0.6 mm, the height of the bosses is 4.5 mm, and the minimal height of the sides is 6.5 mm. All of these can meet the requirements of 3C products. From Fig.4, it can be concluded that the material flow of the long side is better than that of the short side and the corners, the height in the long side centre is the highest, but the corners are the lowest. The constraint wall in punch B can limit the forming height of the long side, and makes the materials flowing to the corners, which can increase the forming height of the four corners. The outlet track in punch B can accommodate redundant materials. Fig.5 shows the experimental results of 3 mm thickness blanks after press forging at 250 C using punch B. It is clear that the height of the side is even. (a) Phenomena of twist and flow through (b) Good formed part (c) The height of the side with different punch displacements Fig.4 Formed parts of 2 mm thick blanks using punch A at 250 C (a) (b) Fig.5 Formed parts of 3 mm thick blanks using punch B at 250 C: (a) Front view of the formed part with materials flow of long side limited by constraint wall; (b) Top view of formed part after trimming Fig.6 Finite element mode l

4 1756 THERMEC Finite element simulation 3.1 FE Model A 3D FE model was established to simulate the press forging process of circular blanks using Deform3D code. According to the symmetry, one quarter of the component was used in the model. The punch and the die were defined as rigid bodies while the blank material was defined as a viscoplasticity body governed by the stress-strain curves obtained from the hot compression tests. The shapes of the model were shown in Fig.6. The deformation was assumed as isothermal press forging process, so the blank temperature was constant in the simulations. Friction conditions at the interface between blank and tools were modeled by Shear Friction Laws, and two friction coefficients were adopted in the simulations to simulate the different experimental lubricating conditions. The specific parameters used in the simulations are listed in Table 1. Table 1 Mechanical properties of AZ31B and process parameters Young s modulus (GPa) 44.8 Density (g/cm 3 ) 1.78 Poisson s ratio 0.35 Friction coefficients between punch-sheet Interface 0.15 Friction coefficients between die-sheet Interface 0.3 Punch speed (mm/s) Comparisons and analysis Fig.7 Comparison of load-displacement between FEM and experiment using punch A (a) FEM Result (b) Experimental result Fig.8 Shape comparison between simulation and experiment using punch A Fig.7 and Fig.8 show that the simulated results agree with the experimental results at 250 C. It means that the value of the friction coefficient for the simulations carried out in this paper is close to the real value and the model is correct. This indicates that FEM simulation can be used accurately to evaluate the approximate punch load and the shape after forming. As shown in Fig.7, the thickness of the box bottom was 0.6 mm when the displacement of the punch reached 1.4 mm,

5 Materials Science Forum Vols and the load was about 550KN. From Fig.8, it can be seen that the height of the four bosses did not reach the depth of the holes in the punch, which is different from the result of the simulation. The reason is that the hole in the punch is airtight. The experimental and simulated results show that the forming height of the long side is higher than that of the other parts of the box; especially the height of the middle part of the long side is maximal. This uneven flow behavior of the materials in the press forging process makes the great difference among the height of the long side, the short side and the corner. The difference can be reduced by using punch B, and the side height can be uniform. The simulated results are shown in Fig.9. The blank thickness is 3 mm and press forging was carried out at 250 C by using punch B, and the load-displacement curve is shown. The thickness of the box bottom was 1.1 mm when the punch displacement reaches 1.9 mm, and the load was about 700 kn. The punch load at the end of press forging is relatively higher than using punch A. So the punch load will be out of the limit of the press to forge the box with the bottom thickness 0.6mm. Fig.9 Simulation of 3 mm blank conducted Fig.10 Temperature effects on punch load evaluated by press forging at 250 C using punch B by FEM (2 mm blanks using punch A) 3.3 Temperature effects on punch load The punch load applied to the blank is significantly influenced by the blank temperature. The effect on punch load was studied by FE simulation (2 mm thick blank forged by using punch A). Fig.10 shows the variations of punch load with punch displacement at temperatures ranging from 250 C to 350 C. Punch load can be reduced effectively with increase of the blank temperature. 3.4 Friction effects on load and formed shapes (a) Effect on minimal height of side (b) Effect on punch load Fig.11 Friction coefficient effects on press forging by finite element simulation

6 1758 THERMEC 2006 Friction has great influences on press forging. The friction coefficients between punch-sheet interface and die-sheet interface were regarded equal to carry out finite element simulation (2 mm thick blank forged by using punch A) and study the effects. Fig.11 is the results of finite element simulation and the effects on press forging. Fig.11(a) shows the effect on punch load, and Fig.11(b) shows the effect on the minimal height of side. It is clearly observed that the punch load and the minimal side height decrease with decrease of the friction coefficient. 4. Summary (1) The rectangular box of magnesium alloy sheets with bosses used for 3C products can be formed by press forging process. The bottom thickness can reach 0.6mm, and the height of boss can reach 4.5mm. (2) The punch with constraint wall and outflow track can reduce the difference of the side height and make side height uniform. (3) Friction has great influences on press forging. Low friction coefficient can reduce the difference of side height and punch load. The blanks can be formed well only the top-surface of the blank is fully lubricated and the bottom-surface is not lubricated or is lubricated a little oly. (4) Increasing forming temperature can reduce punch load effectively. References [1] S.H. Zhang, K Zhang, Y.C. Xu, Z.T. Wang, Y. Xu, Z.G. Wang, Deep-drawing of Magnesium Alloy Sheets at warm temperatures, ICAMT (2004 Malaysia): [2] Y.C. Xu, S. H. Zhang, H.M. Liu, Z.T. Wang, W.T. Zheng,Q. L. Zhang, Y. Xu. Improved Formability and Deep Drawing of Cross-rolled Magnesium Alloy Sheets at Elevated Temperatures, Material Science Forum, Vol (2005): [3] S.H. Zhang, Y.C. Xu, G. Palumbo, S. Pinto, L. Tricarico, Z.T. Wang, Q.L. Zhang, Formability and Process Conditions of magnesium Alloy Sheets. Material Science Forum, Vol (2005): [4] Y.C. Xu, S.H. Zhang, W.T. Zheng, L. Tricarico, G. Palumbo, D. Sorgente. Investigation on the sheet forming of magnesium alloys, Proceedings of the 8 th ICTP, Advanced Technology of Plasticity (2005): , Oct.9-13, 2005, Verona, Italy [5] R. Matsumoto,K. Osakada,Development of Warm Forging Method for Magnesium Alloy, Materials Transactions, Vol.45,No.9(2004):

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